ci: Add CI build for macOS
ci: Add CI build for macOS

Note that this skips the 'make distcheck' stage due to two reasons:

  * GitHub considers macOS cpu cycles to be very expensive, and
    distcheck basically doubles the build cycles
  * For reasons yet unknown, 'make distcheck' on macOS fails with
    the compiler being unable to create an executable.

gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2